The object of the present invention is a device and a method for the controlled generation of vibrations by magnetic means, which can be used as a damper or exciter of vibrations.
In numerous industrial applications, for instance in machining metal by mechanical operations, it is useful to damp vibrations, both free and forced, generated by machine tools.
In the case of cut operations in particular, the reduction of vibrations is a critical factor both in rough-cutting and in finishing. It is desirable to have a high cutting speed, which brings with it an increase in vibrations, but simultaneously maintain high precision of workmanship, which instead requires the limitation of the vibrations generated.
It is common practice to reduce the vibrations caused in a machine tool by means of passive dampers, the simplest of which consists of the employment of masses of considerable weight (plinths) fixed to the faceplate of the same machine tool.
But such means of damping are often revealed to be inadequate in the case of precision and/or high-speed working, since they are not sufficiently effective in reducing the vibrations of the structure and are usually only able to reduce low-frequency oscillations.
Active-agent vibratory devices have therefore been proposed which function according to the xe2x80x9cdestructive interferencexe2x80x9d principle, by which it is possible to reduce and theoretically eliminate the oscillations present in a structure by generating new oscillations in the same structure which have the same frequency as the first, but are opposite in phase.
Devices which operate according to the aforementioned principle, usually comprise means for the generation of vibrations subject to automatic controllers which are based on signals relating to vibrations present in the structure to be stabilised, and which supply the necessary opposite phase oscillation.
For these devices to be efficient, it is necessary that the means of generation of vibrations be very rapid in varying the oscillation produced in conformity with the signals furnished by the control system. For this reason such means of vibration generation usually operate electromagnetically, along multiple axes.
The European patent No. 338,933 describes a device for the generation of controlled vibrations, comprising an internal spherical body placed inside a hollow casing, at least six magnetically-active bearing elements, located internally in the casing, means of detecting the vibrations, elements for detecting the position of the internal body and circuits for the automatic control of the magnetic-bearing elements.
The hollow casing is made integral to the structure to be stabilised by means of mechanical fixing elements, such as screws and bolts, while the internal spherical body, which comprises elements in ferromagnetic material, is suspended by means of the magnetic-bearing elements, so that there is no contact between the casing and the same body.
The control circuit, based on the signals deriving from the means of detection, provides for the modification of the intensity of current circulating in the bearing elements, and therefore the magnetic field generated by them.
This means that the forces of attraction to which the internal body is subject change with time in accordance with the signals generated by the control circuit, causing accelerated motion of the internal body with respect to the casing.
The consequent force of reaction acting on the hollow casing causes a vibration which is propagated to the structure to be stabilised.
In this way the control circuit, which operates according to a feedback process, is able to regulate the vibrations caused by the movement of the internal mass with the objective of reducing the oscillations of the structure to which the casing is attached.
While the employment of an internal mass comprising ferromagnetic material, e.g. one or more ring armatures in ferrous material, simplifies the construction of the device on the one hand, on the other it makes it less efficient. In fact, ferromagnetic material is subject to frequent cycles of hysteresis which involve, beyond the dissipation of energy, also a low response speed to the forces generated by the magnetic bearings and, consequently certain limitations in the operating frequency range of the device.
Furthermore, the forces generated by the magnetic bearings are only of attraction and they accordingly must be sufficiently large to overcome the inertia of the large mass of the internal body.
One objective of the present invention is to realise a device for the controlled generation of vibrations which has a high speed of response to the signals which originate from the means of automatic control.
Another objective of the invention is to supply a device for the controlled generation of vibrations which acts on a wide range of frequencies.
A further objective of the invention is to propose a method for the generation of vibrations which is particularly effective, even in the presence of high operating frequencies.
These objectives are achieved by the device for the generation of vibrations characterised according to claim 1 and by the method for the generation of vibrations according to claim 17.
Further characteristics of the invention are described in the remaining dependent claims.
The device for the generation of vibrations according to the invention presents an external casing, provided with means of connection to a support structure, and a mobile mass housed within the casing. The mobile mass comprises at least one active magnetic element, such as a permanent magnet or an electromagnet or a combination of the two, which cooperate with magnetic means of guidance, which are housed in the internal surface of the casing.
The magnetic means of guidance are slaved to means of automatic control, which are in their turn connected to means of detecting the position and/or the motion of the mobile internal mass.
The employment of the same means of detecting the position and/or the motion of the mobile internal mass produces a device which is particularly simple to install, and which doesn""t present elements external to the casing to be positioned along the support structure.
According to a particular embodiment of the invention, the device is provided with means of detecting the vibrations of the support structure, operationally connected to the means of control. Such means of detecting the vibrations of the support structure could be suitably placed in correspondence to a support base of the device, and isolated from the latter, to guarantee simplicity of installation and, at the same time, avoid unwanted errors of detection.
In particular, combining the means of detecting the position and/or the motion of the internal mass with the means of detecting the vibrations in the support structure proves particularly useful in obtaining high-precision response of the device to variations in the vibrations in the support structure.
In one possible embodiment of the device, the mobile internal mass is substantially cylindrical.
In a further embodiment, the device according to the invention comprises magnetic means of suspension of the mobile internal mass coincident with the aforesaid magnetic means of guidance.
According to another aspect of the invention, the magnetic means of guidance and suspension comprise windings of conductor material, which present cores substantially devoid of ferromagnetic material.
This solution allows the weight of the external casing to be reduced and a higher frequency response speed of the device to be obtained.